1. Technical Field
The present invention is directed to apparatus used in the administration of blood and cardioplegia solution during cardiac surgery.
2. Background Information
Since the early days of cardiac surgery, it has been recognized that in order to provide the optimum surgical conditions when operating on the heart, it is necessary to interrupt the normal operation of the heart. For obvious reasons, an arrested, flaccid heart is preferred during a cardiac surgical procedure over a beating heart with blood flowing through it. Thus, in order to be able to efficiently perform cardiac surgery, it is often necessary to use cardiopulmonary-bypass techniques and to isolate the heart from its life-giving blood supply.
It has been found that many deaths occurring after cardiac surgery are due to acute cardiac failure. At first, it was believed that the heart was simply beyond repair and that the operation had failed to correct the problem. Later, it was discovered that many of these postoperative deaths were due to new, and often extensive, perioperative (during or within 24 hours after the surgical procedure) myocardial necrosis (death of the heart tissue). Furthermore, many patients who survived were found to have suffered myocardial necrosis to a significant degree, thereby resulting in low cardiac blood output.
It is now known that myocardial necrosis occurs because the energy supply or reserve of the cardiac muscle cells is inadequate to supply the needs of the heart. The availability of oxygen dramatically affects the cell's ability to satisfy these energy requirements. For example, anaerobic metabolism of glucose produces two (2) moles of adenosine triphosphate ("ATP") per mole of glucose (as well as harmful acid metabolites), whereas aerobic metabolism of glucose produces thirty-six (36) moles of ATP per mole of glucose. Therefore, one of the primary goals of myocardial preservation techniques during surgery is to reduce myocardial oxygen consumption.
Myocardial oxygen consumption is significantly reduced by stopping the electromechanical work of the heart. The oxygen demands of the beating empty heart at 37.degree. C. are four to five times those of the arrested heart (i.e., 4-5 ml/100-gm/min compared with 1 ml/100-gm/min). Buckberg, G. D., "Strategies and Logic of Cardioplegic Delivery to Prevent, Avoid, and Reverse Ischemic and Reperfusion Damage," 93 The Journal of Thoracic and Cardiovascular Surgery, 127, 136 (January 1987) (hereinafter referred to as: Buckberg, "Strategies and Logic of Cardioplegic Delivery").
The normal heart receives its blood supply through the left and right coronary arteries which branch directly from the aorta. Generally, the veins draining the heart flow into the coronary sinus which empties directly into the right atrium. A few veins, known as thesbian veins, open directly into the atria or ventricles of the heart.
One of the early methods utilized to protect the myocardium during surgery was normothermic perfusion of the empty beating heart. This method was utilized in an effort to maintain the heart, as nearly as possible, in normal conditions during surgery. Although the procedure eliminated the problem of blood flow, dissection and suturing were still difficult to perform because of the firmness of the myocardium and the beating of the heart. Additionally, it was found that a significant amount of damage still occurred to the myocardium even when this procedure was utilized.
A second method which was developed to protect the myocardium was intermittent cardiac ischemia with moderate cardiac hypothermia. This method requires that the entire body be perfused at a temperature of from 28.degree. C. to 32.degree. C., thus slowing all bodily functions, including those of the heart. The heart is fibrillated before aortic cross-clamping to stop the beating. The surgeon can then operate for approximately fifteen to twenty-five minutes, after which time the heart beat is necessarily resumed for three to five minutes. This procedure proved to be an inefficient method for performing operations and had many attendant dangers, not the least of which is subjecting the heart to multiple fibrillations.
A third method which has been utilized is profound hypothermic cardiac ischemia. This method requires that the temperature of the heart be lowered to about 22.degree. C. by the infusion of a cooled perfusate and/or by filling the pericardium with cold saline solution. One of the major disadvantages of this technique is that the heart continues to fibrillate, exhausting the heart's stored energy. As a result, the heart becomes acidotic, which over time causes irreversible muscle damage.
A fourth method which has been developed to preserve the myocardium during surgery is the infusion of a cold cardioplegic fluid to cool and stop the beating of the heart. After the initial infusion, the heart is reperfused approximately every thirty (30) minutes to maintain the cool, dormant state of the heart.
Cardioplegia, which literally means "heart stop," has proved quite advantageous. Cardioplegic solutions, typically containing potassium, magnesium procaine, or a hypocalcemic solution, stop the heart by depolarizing cell membranes. Cardioplegia may be administered in an antegrade manner (through arteries in the normal direction of blood flow), in a retrograde manner (through veins opposite the normal blood flow direction), or in a combination of retrograde and antegrade administration.
The most recent method for preserving the myocardium during surgery utilizes continuous warm blood cardioplegia. Warm oxygenated blood cardioplegia has certain theoretical advantages over cold cardioplegia, such as continuously supplying oxygen and substrates to the arrested heart while avoiding the side effects of hypothermia. Salerno, Thomas A. et. al., "Retrograde Continuous Warm Blood Cardioplegia: A New Concept in Myocardial Protection" 51 Annual of Thoracic Surgery 245 (1991).
The use of warm blood cardioplegia to protect the myocardium has proven the most advantageous method of those used to date. Warm blood cardioplegia satisfies the myocardial metabolic demands of the arrested heart by providing warm blood with oxygen contents that exceed 15 ml of oxygen for every 100 ml of blood.
Studies reveal that functional recovery is significantly improved when warm blood cardioplegic solution is used. A study by Magovern and co-workers reveals that oxygen delivery at temperatures above 20.degree. C. is significantly improved over lower temperatures. Engleman, Richard M., "Retrograde Continuous Warm-blood cardioplegia," 51 Annual of Thoracic Surgery 180 (1991). Cardioplegia may be induced immediately after extracorporeal circulation has begun, provided that the pulmonary artery is collapsed to attest to the adequacy of venous return.
Warm-blood cardioplegia is often initiated by placing an aortic antegrade cardioplegia cannula followed by the placement of a retrograde coronary sinus catheter. Typically, a high-potassium (50 mEq KCl) warmblood cardioplegic solution is infused through the antegrade cannula to rapidly induce cardiac arrest. Following diastolic arrest, it is typical to switch perfusion from a bag containing the high-potassium cardioplegic solution to a bag containing a low-potassium (30 mEq KCl) solution, which is infused continuously. Reducing the amount of potassium perfused into the heart lessens the risk of overloading the heart tissues with potassium, which overload makes it more difficult to restart the heart after surgery. Overload of potassium could also result in ischemic periods in the heart muscles, or cause other problems following surgery.
Although the use of high-potassium cardioplegic solution to arrest the heart followed by use of a low-potassium solution is widely practiced, as the best method presently available, it is known that this practice is not altogether satisfactory. In order to insure that enough potassium is administered to maintain a state of heart stasis, the low-potassium solution still introduces too much potassium for many patients. This causes the problems mentioned above, and further results in the administration of a higher volume of fluids than necessary if one were to have better control over the amount of potassium solution infused into the patient. Excess infusion of liquids forces the patient's renal system to work harder to restore equilibrium, which can be an unreasonable strain to a patient as ill as those typically requiring open heart surgery.